Mirror Blanks
These were ordered at the end of 2007 from Hextek Corp, Tucson, AZ. In June 2008 we were able to see the first blanks at the Hextek workshop. We expect about one set of mirror blanks each month from now on. The mirrors are initially fused flat, with cylindrical glass tubes separating the top and bottom glass plates. Hextek provide mirror blanks which are as stiff as solid blanks of the same size, but much lighter - each primary weighs 160 lbs, each secondary weighs 12 lbs. There are 108 cells in the 1-meter blank which is 7 inch thick. The blanks have also been heated and slumped so the front surface is close to a sphere that best matches the desired curvature of the final mirror.
The secondary has 48 cells and is 13 inch diameter by 2 inch thick. Each cell pocket has a hole through the non-reflecting surface that allows airflow across and inside the mirror. The large ratio of surface area to mass allows the glass to quickly cool to ambient air-temperature, and thus avoids image degradation that could be caused by mirrors remaining hot from daytime heating.
Polishing
In July 2008 we signed a contract with Lytkarino Optical Glass Factory to polish the 1-meter optics. The first set of mirror blanks will go to LZOS in Russia in September, followed by shipments every 6 months. The first set of finished and coated mirrors is expected in early 2009.
Telescope Schedule
We plan to have the first telescope ready for mechanical testing at Santa Barbara by the end of 2008, and have the first prototype telescope working on the sky in Santa Barbara with an imager (not the final version) early in 2009. There will be a period of testing the telescope system: mechanics, drives, optics, control system, camera and scheduling, then we plan to deploy the first fully functioning 1-m telescope in a roll-off enclosure, with control system, 4K CCD and associated equipment, by the end of 2009. Observation requests that have been approved by the Time Allocation Committee will be entered onto a network queue: automatic scheduling software will decide when each observation will take place, and on which telescope.
Image Quality
The 1-m telescopes are designed to work remotely and reliably, and provide accurate photometry. The figure shows the optical quality calculated on-axis, at an edge and corner of the CCD. Each box is scaled to represent a single CCD pixel (0.4 arcsec). Image quality degrades a little at the autoguider field which is 95-mm off axis. The box there shows a typical binned autoguider pixel of 0.8 arcsec. A medium resolution spectrograph (one for each cluster) will be fed from a 3-arcsec diameter fiber for each telescope from this off-axis area.
The main requirement is that neither the telescope (optics, tracking, local thermal environment) nor detector system (autoguiders, CCD, filter wheel) should noticeably degrade the intrinsic site image quality. Our telescopes will be deployed at existing and well-serviced observatories, where we typically expect image quality of 0.8-1.3 arcsec, so the intrinsic image quality will always exceed the optical quality of the telescope.
Secondary Mirror Support
The mirrors are made of borosilicate glass (quartz), and do expand with temperature. The secondary support is an Invar hub, with Nickel content adjusted to best match the expansion coefficient of the glass. Prior to polishing, it is epoxied to the back surface of the secondary. A cylinder protrudes through the central hole to provide an air passage for clean dry air to pass over the reflecting surface in humid conditions, and to support an alignment fixture. The central hole is preserved and can be used to minimize thermal background radiation for a possible future infrared camera.
Primary Mirror Support
Finite element analysis showed that a central hub support could work for the primary mirror also, but the required hub would be large and heavy. Therefore the primary is supported on a traditional 18-point whiffle tree, with an 18-point radial support system operating on the lower plate of the mirror. The cut-away figure shows the primary mirror in its cell, which also forms the declination axis for the telescope. The baffle is supported from the mirror cell and does not touch the mirror. The optical prescription includes a doublet lens corrector for good wide-field images, which can be seen supported in the central hole of the primary.
